1 / 29

Comet Ephemerides with Geometry and Visibility Info

Comet Ephemerides with Geometry and Visibility Info. by Steve Albers. DEC, RA, Distance from Sun and Earth Magnitude, Tail Length, Tail orientation Phase Angle, Elongation Rise/Set Times, Time/Altitude for best viewing Visibility compared to naked eye, binocular thresholds

druce
Download Presentation

Comet Ephemerides with Geometry and Visibility Info

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Comet Ephemerides with Geometry and Visibility Info by Steve Albers

  2. DEC, RA, Distance from Sun and Earth • Magnitude, Tail Length, Tail orientation • Phase Angle, Elongation • Rise/Set Times, Time/Altitude for best viewing • Visibility compared to naked eye, binocular thresholds • expressed as “effective” or corrected magnitude • Output computed daily or more frequently Ephemeris Output

  3. Effective Magnitude Adjusted from Actual Magnitude • compare to naked eye 6.0 threshold • Effective Magnitude Adjustments • extinction • sky brightness (moonlight, twilight, daylight at best observation time) Visibility Computation

  4. Nighttime • moderate light pollution assumed, increasing near horizon • increased by scattering/glare from the moon • Twilight • empirical relationship of limiting magnitude and comet/sun altitude difference • Daytime • scattering/glare based on elongation from sun, comet altitude and solar altitude Sky Brightness Computation

  5. Internet Demo? http://laps.noaa.gov/cgi/albers.homepage.cgi (optional)

  6. PANSTAARS reaching (barely) naked eye magnitude limit in March PANSTAARS reaching (barely) naked eye magnitude limit in March • Ikeya-Seki visible naked eye in Japan within hours of perihelion, otherwise head was likely invisible naked-eye (despite impressive tail) • ISON orbit similar to Ikeya-Seki, though will be dimmer and may never reach naked-eye brightness • Hale-Bopp longest most visible comet in last 50 years • McNaught (2007) visible naked eye from Longmont area in bright twilight, and in binoculars during the daylight Visibility Examples

  7. Sky Brightness Computation • It’s fine to calculate sky brightness and limiting magnitude at individual points in the sky for an ephemeris, however… • Wouldn’t it be nice to show an image of sky brightness over the entire sky??

  8. Simulated All-Sky Images Compared with the LAS All-Sky Camera by Steve Albers, Vern Raben, and the NOAA LAPS Group

  9. 3-D Gridded Cloud Analyses (or forecasts) • Cloud liquid, ice, rain, snow,hail • NOAA’s LAPS model (developed at ESRL) • Locations of Sun, Moon, Planets, Stars • Specification of Aerosols (haze) • Specification of Light Pollution • Specify Vantage Point • Latitude, Longitude, Elevation Simulation Ingredients

  10. LAPS Cloud analysis First Guess  METAR METAR METAR OAR/ESRL/GSD/Forecast Applications Branch 10

  11. Illumination of clouds, air, and terrain are pre-computed • Sky brightness based on sun, moon, planets, stars • Ray Tracing from Vantage Point to each sky location • Scattering by Intervening Clouds, Aerosols, Air • Terrain shown where its along the line of sight • Physically and Empirically based for best efficiency Visualization Technique

  12. Overall correction based on optical axis centering, spherical rotation, and radial lens “distortion” Image Navigation • Need to rotate around Lambda Draconis? • Except that near horizon offsets are just in azimuth (zenith rotation)

  13. Cloud Illumination (and scattering) Cloud Illumination Example

  14. Nighttime Clouds (and stars)

  15. Source can be sun or moon • Rayleigh Scattering by Air Molecules (blue sky) • Minimum brightness 90 degrees from light source • Blue-Green sky color near horizon far from sun • Mie Scattering by Aerosols (haze) • Brighter near the light source (aureole) • Added sky brightness from planets, stars, light pollution, airglow Background Sky Brightness

  16. Daylight Clear Sky

  17. Nighttime Comparison

  18. Cloud shadows in clear air can show crepuscular rays • Brightness and color changes shown during twilight • 3-D orientation of Earth’s shadow considered • Secondary scattering needed to reduce contrast in Earth’s shadow that appears opposite the sun Clear Air Illumination

  19. Twilight Comparisons

  20. Twilight Comparisons

  21. Topography data allows showing mountains near the horizon • Terrain Albedo (e.g. a dark forest) • Adjusted by cloud shadows • Show snow cover (future enhancement) • Terrain can be obscured by intervening clouds, haze, or clear air (very long distances) Terrain Illumination

  22. Trace from viewer into sky at ~1x1 degree grid • Ray path travels through clear air, aerosols, clouds, and may hit terrain • First estimate is clear sky value (background sky) • Scattered by clouds (can show up either bright or dark) • depends on optical depth of cloud and elongation from sun, as well as pre-computed cloud illumination • Cloud/Aerosol scattering can obscure distant terrain Main Ray-Tracing Step

  23. Mie scattering phase function means thin clouds are brighter near the sun (silver lining), cloud corona • Thick clouds are the opposite, being lit up better when opposite the sun • Rayleigh scattering by clear air can redden distant clouds • Future enhancement would be to add rainbows & halos • (with clouds/precip at specific elongation angles) More on Cloud/Precip Scattering

  24. Cylindrical grid (panoramic view) can be calculated at either 1x1 or 0.5x0.5 degree spacing • Currently just shows at and above the horizon • future enhancement to show below the horizon • Convert to polar grid (shown here) • good for overhead views, and for camera comparison Final Display

  25. Cylindrical Panoramic View (½ degree resolution)

  26. Example Animation #1

  27. Example Animation #2

  28. Internet Demo of All-sky Web page? (optional) http://laps.noaa.gov/allsky/allsky.cgi

  29. What’s next? The sky is the limit!

More Related